This invention relates to sheet material which exhibits release properties toward a wide variety of normally tacky and pressure-sensitive adhesives.
For many years manufacturers of inherently tacky mastics or adhesives have protected the compositions with such anti-stick, or "abhesive" materials as wax-coated paper, plastic films, plastic-coated paper, starch-impregnated fabrics, and extremely complex polymer systems.
Such polymer systems have found particular application as coatings on the back surface of normally tacky and pressure-sensitive adhesive tape wound in roll form, where they function as so-called low adhesion backsizes (LAB), facilitating use of the tape and preventing inadvertent transfer of the adhesives to the back surface. The force to separate the adhesive from an LAB coating is less than the force required to separate the adhesive in the absence of the LAB but is typically more than 80 grams per centimeter width. For exemplary polymeric low adhesion backsizes, see U.S. Pat. Nos. 2,532,011, 2,607,711, 2,876,894, 3,318,852 and 3,342,625.
For some products, e.g., labels or large adhesive-coated sheets sold in other than roll form, it is desirable to have a protective release liner to which normally tacky and pressure-sensitive adhesives adhere very weakly; U.S. Pat. Nos. 3,061,567, 3,518,325 and 3,671,485 are illustrative of such products, all of which are based on a silicone coating of some type. The force to separate a pressure-sensitive adhesive from a silicone-treated surface is typically in the range of 2-10 grams per centimeter of width. Generally speaking, it has previously been extremely difficult to obtain materials having release properties intermediate those of the LAB-type polymers and those of the silicones without sacrificing some other essential property. Recognizing the desirability of a release coating having properties intermediate the 10 grams per centimeter afforded by the silicones and the 500 grams per centimeter of typical low adhesion backsizes, attempts have been made to modify silicone polymers by blending or reacting them with other less effective release materials; see, e.g., U.S. Pat. Nos. 3,328,482, 3,527,659, 3,770,687 and 3,891,745. Some of the resultant hybrids so contaminate a pressure-sensitive adhesive that it loses its tack, while others gradually react with pressure-sensitive adhesives in such a manner that they cannot be separated after remaining in contact for an extended period of time. Many compositions are difficult to reproduce with consistency, exhibit changed release properties as the silicone gradually migrates to the surface or require cure temperature so high that they adversely affect the substrate on which they are coated.
One of the best products of the type described in the preceding paragraph is shown in U.S. Pat. No. 3,957,724, where the release agent is the reaction product of an isocyanate-terminated moiety and a liquid hydroxyl- or amine-containing organosilicone. This product, however, is effective with only a few specific adhesives.
Still another approach to obtaining release surfaces having properties intermediate those of silicones and the more conventional low adhesion backsizes is shown in U.S. Pat. No. 3,503,782. This patent discloses broadly the preparation of a release surface by uniformly applying a first release agent coating to a substrate so as to entirely cover the surface and then applying a second release agent to only a part of the area covered by the first release agent. The only examples shown involve the use of a conventional silicone as the first release agent and a filled silicone as the second release agent. This fact is not surprising, since most low-adhesion backsizes do not adhere to a silicone surface, and if a silicone release agent is applied over a conventional LAB, it tends to wet so excessively that it covers the entire surface, in effect yielding a release surface which is silicone alone.